AND ITS APPLICATION TO THE DETERMINATION OF COLOUR SENSATIONS. 335 



The reflection from the semi-silvered mirror M! falls on a silvered mirror, M 2 , which 

 reflects the beam in such a direction that it falls on B, the image of the spectrum 

 being thrown on D 2 , in which are slits, S 3 . The image of P, is thrown on B by the 

 lens L.v A beam of white light is reflected from the face of P] by M : , (\vhich may be 

 either a silvered mirror or plain) and is also focussed on B, so that we have the 

 patches from both spectra and from the white light falling over one another on B. 

 By means of rods correctly placed, a colour or colours from either spectrum can be 

 isolated and be mixed with anv proportion of white by using sectors as shown. 

 There are slides carrying the slits at D! and I) 2 , and to them are attached trans- 

 parent scales, [n the case of 1)^ a beam of white light falls on the mirror M 5 , as 

 shown, and passes through the transparent scale at "<," and a lens X throws a 

 magnified image of the graduation on a distant white screen, on which a zero mark 

 is drawn. This enables the transparent half-millimetre scale to be read to a tenth 

 of that unit. In a similar way the scale at " <t " is magnified by X' by a beam of 

 light falling on M,. When the scale readings are not required, the sources of light 

 illuminating them are covered up. 



Again the small lenses A 1 and A" are mounted in a sliding arrangement and can be 

 moved in front of lenses L., and L ti . When a sl.it is drawn in front of A 1 or A 1 ' the 

 image of the aperture is magnified on a distant screen, carrying a scale, and the 

 width of the slits can be accurately ascertained by noting on such scale the reading 

 of the breadth (say) of -J- millim. width of slit. This is the instrument with which 

 the following measures were made. 



(:j.) I* t/icri' <i 4t/i Xcitx'ttinii. in tin- Vinli-t .' 



As in my previous investigations, the red at the red lithium line was used as 

 exciting only the red sensation, and the violet at X4100 was also employed as a 

 .provisional sensation, since it excited only the blue and the red sensations. 



Since my last paper on the subject was published, Brucn, in his paper in the 

 'Phil. Trans.' (B, vol. 191, 1899) has given it as his opinion that besides the red, 

 blue, and green sensations there is a 4th sensation excited by the violet. Before 

 using the violet as a provisional sensation, it became necessary to ascertain if this 

 4th sensation really existed, and various experiments were made with this object. 

 From the first I was sceptical as to the 4th colour sensation, as it appeared to 

 me to be unnecessary, and was a departure from the simplicity with which nature 

 usually works. Amongst the experiments tried was that of fatiguing one of my 

 own eyes with strong red light, and by a simple artifice immediately afterwards 

 viewing a patch of violet light, keeping the uufatigued eye closed. The violet 

 became a bright blue, whilst to the unfatigued eye it was of its natural violet hue. 

 Not satisfied with my own vision, 1 got several unbiased persons to repeat this 

 experiment, and they invariably stated that the patch became blue. A red-blind 



